"You can't shoot the animal after legal shooting hours if you found it alive, you could expose yourself to a whole bunch of hunting violations (think spotlighting, poaching..serious stuff)
Technically if you found the animal alive you are legally obligated to let it continue to suffer all night long and shoot it during legal hours the next morning, or call a WDFW officer to seek permission."
I once shot an elk and wounded it right at the end of legal shooting time. I tracked it for over an hour, after legal time, with a fliashlight and rifle. I found it by flashlight and shot it.
Legal or not it was the right thing to do.